Polyvinyl chloride-clad urethane foam is used extensively as trim components in automobiles. The styling of the contemporary automobiles radically expands the amount of PVC-clad polyurethane foam components exposed to the sunlight. The rakish windshield angles allow the top of the dashboards to attain temperatures as high as 260.degree. F. Such prolonged exposure to sunlight leads to discoloration (staining) and eventually cracking and splitting of the PVC skin. This extreme thermal and ultraviolet stress is exacerbated by the volatile or fugitive amines in the polyurethane foam.
In order to obtain the sweeping angles and curves typical of present day automotive interior trim components such as dash pads, consoles, arm and head rests, the PVC skins have been made thinner with a higher level of plasticizer.
One of the most significant contributors to discoloration is believed to be the thermally induced vaporization of the amine catalysts used to make the polyurethane foam. These catalysts are believed to migrate into the PVC and dehydrohalogenate the PVC forming polybutene, a black, brittle material.
The industry has attempted to improve the PVC staining resistance of the trim parts. One approach is to paint the part after the foaming composition has been poured onto the PVC skin. This is an expensive procedure prone to high rework rates that increase the costs. Another attempt has been to use non-fugitive amine catalysts. The non-fugitive catalyst contains one or more active hydrogen groups in the molecule that react with the isocyanate moiety and are permanently fixed into the foam. These catalysts produce a reasonable solution to the staining problem but create another problem which is the production of high internal pressures within the foam as it flows through the instrument panels. The result of this high internal pressure is to create voids under the PVC skin. These voids are visible and tactile.
Another attempt to solve the PVC staining problem is to use vinyl amine scavengers such as chlorinated phosphate esters. These materials have to be included in the isocyanate part of the polyurethane mix because they would deactivate the amine catalysts if added to the polyol component containing the amine catalyst. This approach has shown only marginal success.